The ability to regulate hematopoiesis and to maintain hematopoietic balance is critical to the welfare of an organism, whether it is to meet additional demands to combat invading pathogens, or to re-establish the hematopoietic compartment subsequent to myelo-ablative events. Aging is accompanied by a general decline in hematopoietic capabilities, contributing to an increasing susceptibility to infections and to autoimmune conditions. A key parameter in the overall maintenance of the hematopoietic compartment is the residency of hematopoietic stem and progenitor cells (HSPCs) in the appropriate supportive marrow niches. Sialylated glycans participate in diverse cellular adhesive processes impacting multiple aspects of immunity and trafficking; however, little is known of the roles glycans play in early hematopoietic processes. Our laboratory has recently uncovered a novel biologic function for the sialyltransferase, ST6Gal-1, in the regulation of HSPC proliferation. There is compelling evidence that HSPC surfaces can be remodeled by extracellular ST6Gal-1, in a glycosylation pathway divergent from the canonical ER/Golgi-based pathway. The data point to the novel concept that extracellular glycosyltransferases generated from distal sources can function as systemic factors in regulating hematopoiesis, putatively by the extracellular or extrinsic sialyl-modification of HSPC surface components. There are 4 Specific Aims. The first is to evaluate the impact of dysregulated ST6Gal-1 expression in the bone marrow hematopoietic compartment through the use of mice strains that differ only in the way they express ST6Gal-1. The second aim is to evaluate the relative contributions of the extrinsic and the canonical ER/Golgi-based pathways of ST6Gal-1 in sialylation of hematopoietic cell surfaces, with the ultimate aim of identifying the target molecules of extrinsic ST6Gal-1 action. The mechanism by which HSPCs are regulated by ST6Gal-1 will be the focus of Aim 3, through analysis of HSPC-stroma adhesion under static and flow-sheer conditions, and ST6Gal-1 impact on intracellular signaling pathways. Aim 4 will evaluate the long-term impact of dysregulated ST6Gal-1 expression on hematopoietic capacities. The overall goal of this project is to understand the precise contribution and mechanism of ST6Gal-1 in the maintenance of hematopoietic functions, ultimately to yield glycan engineering strategies for effective modification of hematopoietic function.